EXAFS and XANES measurements will be used to characterize the structure of Mn active sites in Mn catalase, arginase, and the photosynthetic oxygen evolving complex. Recent work includes the demonstration that the Mn-Mn interaction is detectable in Mn catalase but not readily detectable in Mn-substituted hemerythrin, Mn-substituted ribonucleotide reductase, or arginase. For the oxygen evolving complex, we demonstrated a spectific reversible structural change in the presence of fluoride.